Ink jet printing is performed by discharging ink droplets from a print head to a substrate. The droplets are ejected through orifices or nozzles in the print head and are directed to the substrate to form an image thereon. In contrast to many other types of printing, there is no contact between the printer and the substrate in ink jet techniques.
Most of the ink jet printers known in the art may be characterized as either continuous, thermal, or impulse devices, depending upon the mechanism by which the ink droplets are directed to the substrate. In continuous ink jet systems, an essentially uninterrupted stream of ink is ejected from a nozzle and breaks up into droplets. The droplets bear an electric charge so that they can be deflected by an applied electric field which is modulated according to the particular image to be recorded. The electric field directs the droplets toward either the substrate or an ink re-circulating reservoir. The inks employed in conjunction with continuous ink jet systems typically comprise a colorant such as a dye or pigment, an electrolyte to facilitate droplet deflection, and a liquid vehicle to dissolve or disperse the colorant and the electrolyte. While the vehicle in many continuous-type inks comprises water, U.S. Pat. No. 4,142,905, in the name of Cooke, disclosed a water-free ink composition consisting essentially of a low molecular weight glycol, an inorganic salt electrolyte, and a dye which is soluble in the glycol.
With so-called “impulse” or “drop-on-demand” ink jet printers, image formation is controlled by changing the rate of energization of a piezoelectric transducer rather than by modulating an applied electric field. Ink is stored in the print head or nozzle until it is necessary to form an image on the substrate on demand. The printer is then activated to apply pressure to the ink and discharge a selected number of discrete ink droplets toward the substrate. These ink droplets need not bear an electric charge. Accordingly, impulse ink compositions can be free of corrosive substances such as water and electrolytes which continuous ink jet inks often comprise.
However, impulse inkjet printers present a number of problems which are not encountered in continuous ink jet systems. For example, unlike continuous inkjet printers, impulse printers typically are maintained in a stand-by or temporarily inoperative mode between printing cycles (often called “startup”). Thus, the ink is allowed to stand, often at elevated temperatures (e.g., about 40 to about 50° C.) leaving it to evaporate and solidify in the discharge orifices of the print head. Evaporation typically leads to changes in ink properties such as viscosity and results in poor print quality. Accordingly, subpulsing is often used to prevent or overcome the problems associated impulse-type ink jet printers when inactive for a period of time. Accordingly, there is a need for impulse-type inks that remain stable during printer downtime and do not require subpulsing for overcoming evaporation or drying effects.
Numerous ink compositions for impulse ink jet printers are known in the art. However, many of these inks are not suitable for bar code printing applications on porous, non-porous, or fibrous substrates. As will be appreciated by those of skill in the art, an ink applied to a substrate such as paper will tend to migrate or wick along the fibers of the paper until the wicking forces are countered by the ink viscosity. The extent to which the ink wicks will be dependent upon both its viscosity and the porosity of the substrate. Where highly porous substrates such as Kraft paper or corrugated cardboard are employed, many inks tend to wick excessively, leading to blurry, ill-defined printed images. One approach to clearer, more well-defined print images has involved the employment of a rapidly evaporating ink composition. However, rapid evaporation of the impulse-type ink often leads to clogging of discharge orifices during stand-by periods. Moreover, such rapid evaporation compositions are less desired because they commonly contain volatile organic components (VOCs) that can be harmful to the environment and human health. Another approach to clearer, more well-defined print images has involved the use of a pigment as a colorant instead of a dye. Where pigments are used as the colorant, the particle size employed must be small enough to achieve reliable performance in the printhead. Water-based pigment dispersions are known in the art that satisfy the printhead performance requirement, however, the reliability of these dispersions in the present application are unknown. In addition, for water-based pigment dispersions, the current state of the art limits the driving frequency of the printhead to between 3 and 8 kHz, resulting in a slower printing operation and through-put. Moreover, water-based pigmented ink systems have two major drawbacks. First, they are unreliable as they tend to settle during storage and in ink reservoirs. Second, water-based pigmented ink systems—especially when the particle size of the pigment gets smaller—tend to entrap more air which results in inconsistent jetting of ink drops.
In certain applications, it is necessary that the image created by an ink jet printer possess a relatively intense threshold color. For example, many optical character reading devices cannot read images unless they possess a minimum color intensity. Those skilled in the art will recognize that bar code images typically must possess a good print contrast signal (PCS) (preferably greater than about 90 percent) to be machine readable. However, many of the known techniques for increasing the color intensity of an ink—such as increasing the concentration of the colorant—often adversely affect important ink properties such as viscosity, surface tension, and stability.
Often, solvent-based ink jet inks contain various amounts of plasticizer to control ink properties such as viscosity and/or surface tension. Potentially harmful phthalate-based plasticizers, widely used for example in plastic products and cosmetics, have recently come under scrutiny as toxins to which people have been exposed at greater levels than previously thought. For example, dibutyl phthalate (DBP) has been widely detected in people. While the effects of DBP exposure are not known, the compound has recently been subject to regulation in Europe. With the possibility of more restrictions on the use of DBP, there is a current need for inks that do not contain DBP or other phthalates, yet possess the desired characteristics for ink jetting.
The inks and processes described herein address the need for solvent-based impulse-type ink jet ink composition capable of producing clear, well-defined, color-intense images on even porous and non-porous substrates. Further addressed by the present invention is the need for inks that remain stable and are resistant to drying during printer downtime. Even further addressed by the present invention is the need forgettable inks that do not contain phthalates.